1. Field of the Invention
The present invention relates to a mobile station included in a mobile communication system such as a mobile telephone or a portable telephone, and more particularly to a mobile station which is arranged to autonomously detect an abrupt change of the quality of a signal received from a peripheral cell and then immediately start to search peripheral cells.
2. Description of the Related Art
A third-generation mobile communication system is planning to employ a CDMA (Code Division Multiple Access) system as a radio access system. The CDMA system is arranged to execute a soft hand-over operation for synthesizing a radio signal from a base station of a current cell (where the mobile station is located) with a radio signal from a base station of a new cell (where the mobile station is moved) as a hand-over when the mobile station is moved from the current cell to the next cell. In order to execute the hand-over, the mobile station is required to do a peripheral cell search for searching for a base station of the next cell.
Each base station is constantly transmitting a radio channel called a perch (or pilot) channel. The perch channel is transmitted through the use of a diffusion (or spreading) code which is peculiar to that cell (base station). Hence, each base station notifies the mobile station of the diffusion code used in that cell on the perch channel. The mobile station operates to periodically measure the ratio of a target signal to interference noise (SIR: signal-to-interference ratio) of the radio channel (the perch channel of the peripheral cell) that corresponds to the notified diffusion code and then notifies the network of the measured result. Herein, the SIR has the same meaning as the "received signal quality", which will be often used below. This operation is called a peripheral cell search. The network determines to which cell the mobile stations is moving.
The action of the conventional mobile station about executing the peripheral cell search will be described with reference to FIG. 1
(Step 1)
The mobile station (terminal) receives the perch channel, obtains the information about diffusion codes used by the peripheral cells, and stores it.
(Step 2)
The mobile station sets a cell search execution period .DELTA.t to a timer. When the timer is counted up, the operation goes to a step 3.
(Step 3)
A reverse diffusion (or de-spreading) process is executed by sequentially using the stored diffusion codes for the peripheral cells, for the purpose of measuring the SIR of each cell.
(Step 4)
The mobile station notifies the network of the result of the peripheral cell search. Herein, the description is explained in the case where the result is reported to the network each time the peripheral cell search is executed. However, the report is not necessarily required for each peripheral cell search. After the result of the peripheral cell search is reported to the network, the operation goes back to the Step 2.
As described above, the mobile station executes the peripheral cell search at a period .DELTA.t and reports the received signal quality from the peripheral cell to the network. If the network determines the mobile station is to be shifted to the peripheral cell based on the report about the received signal quality, the network gives an instruction of a soft hand-over with a target base station (where the mobile station is to be moved) to the mobile station.
However, the foregoing peripheral cell search is executed to measure the received signal quality from the peripheral cell at each period .DELTA.t. Hence, if the received signal quality from the peripheral cell is abruptly changed, disadvantageously, in the worst case, the abrupt change cannot be detected until .DELTA.t is passed. The abrupt change of the received signal quality from the peripheral cell may take place when a base station is moved from a place located out of sight, concretely, behind a building to a place located in sight with movement of the mobile station or when continuous changes take place from one base station located in sight to another one in the case of a mobile station moving in a mountain area.
In order to detect the change of the received signal quality from the peripheral cell as soon as possible, a method may be taken for reducing .DELTA.t. This method, however, requires the mobile station in communication to execute the peripheral cell search with the reverse diffusion at a short period. This peripheral cell search thus increases the processing amount of the mobile station and thereby increases the electric current consumption which results in reducing the using time of the mobile station.
On the other hand, a method for dynamically tracing a mobile station in a cellular network is disclosed in U.S. Pat. No. 5,390,234. In this method, only after the mobile station is turned on and moved by a minimum distance, does the mobile station gives a report to the base station. If the base station needs to communicate with the mobile station, the mobile station is searched only close to the last cell reported by the mobile station to the base station. However, this method provides no capability of coping with an abrupt change of the received signal quality from the peripheral cell.
Further, JP-A-3-268697 discloses a mobile radio communication system in which the mobile station itself constantly monitors the radio line quality based on the received field strength and a code error rate of a demodulated signal while the mobile station is moving over a radio zone. When the mobile station detects a degradation of the received signal field strength or the code error rate owing to the movement over the radio zone, the mobile station sequentially compares one base station having a frequency for the current zone through an empty time slot with another base station in light of the field strength of the signal, for the purpose of determining the signal having the strongest field. The mobile station transmits the frequency information of the determined signal to the base station that is communicating with the subject mobile station, for requesting a hand-over. Then, the base station transmits a message for requesting the hand-over to a control station. In response to the message, the control station performs the hand-over. However, this system enables to switch the channel only when the average received signal level is abruptly lowered. Conversely, if the average received signal level is abruptly enhanced, it means that the mobile station is located in the area where lots of obstacles are located. In this area, the mobile station may utilize a more favorable base station. Regrettably, for coping with such a case, the system does not consider selection of a more favorable base station.
Further, JP-A-5-91038 discloses a system for switching channels of mobile communication, in which system, when the variable width of the received signal level stays in a predetermined range, the channel switching level is set to a higher value and when the average received signal level is abruptly lowered, the channel is quickly switched. This system, however, does not have any suggestion of selecting a more favorable base station when the average received signal level is abruptly increased.
Moreover, JP-A-7-203517 discloses a hand-over system having the function of identifying whether a radio base station in communication and a radio base station adjacent thereto are an indoor radio base station or an outdoor radio base station and the function of measuring the field strength received from the adjacent radio base station and a received field strength variation per unit time. In this system, when the mobile station is communicating with the indoor radio base station and the received field strength and the received field strength variation per unit time from the adjacent outdoor radio base station become more than their predetermined values, the mobile station requires a channel change to the outdoor radio base station irrespective of the received field strength from the indoor radio base station in communication. This system, however, may select the base station that does not have the best received field strength or SIR.
As another prior art, JP-A-8-223107 discloses a channel switching determining device for a mobile radio terminal, in which device request for switching channels are suppressed to a minimum while the mobile radio terminal is moving around an office area with lots of buildings, for the purpose of keeping the communication stable. This device is arranged so that the mobile radio terminal requires to switch communication channels when an average value of bit error rates is inferior to a predetermined value and an average value of received field strengths is lower than a predetermined value. However, this device is not able to cope with an abrupt change of the received signal quality for a period which is shorter than the period when the average value of the bit error rates is measured.